For determining the fill level of a medium in a container, measuring systems are used, which measure various physical variables. On the basis of these variables, the desired information concerning fill level is subsequently derived. Besides mechanical samplers, capacitive, conductive or hydrostatic measuring probes are used, as well as detectors working on the basis of ultrasonic, microwave or radioactive radiation.
In a large number of areas of application, for example in the petrochemicals, chemicals and foods industries, highly accurate measurements of the fill level of liquids or bulk goods in containers (tanks, silos, etc.) are required. Therefore, in such cases, sensors are increasingly being used, in which short, electromagnetic, high-frequency pulses (TDR-method or pulse-radar method) or continuous, frequency-modulated microwaves (e.g. FMCW-radar method) coupled onto a conductive element, or waveguide, and guided by means of the waveguide into the container in which the fill substance is held. Choices for the waveguides include the known variants of Sommerfeld, Goubau or Lecher.
Since, in an especially preferred embodiment, the invention is adapted for TDR (Time Domain Reflectometry) fill level measuring device, the manner in which such a device works will be explained here in somewhat more detail. From a physical point of view, in TDR measuring devices, the effect is used, wherein, at the interface between two different media, e.g. air and oil, or air and water, due to the abrupt change (discontinuity) in the dielectric constants of the two media, a part of the guided, high-frequency pulses, or the guided microwaves, as the case may be, is reflected and conducted via the conductive element back into a receiving apparatus. The reflected part (the wanted echo signal) is, in such case, greater, the greater the difference in the dielectric constants of the two media. On the basis of the travel time of the reflected part of the high-frequency pulses, or the CW-signals (echo signals), as the case may be, the distance to the upper surface of the fill substance can be determined. With knowledge of the empty distance of the container, the fill level of the fill substance in the container can be calculated. If an interface determination is to be performed, then the position of the interface can be determined on the basis of the measurement results.